The present invention relates to a scheduler for use in processor devices and other agents.
As is known, many modern computing systems employ a multi-agent architecture. A typical system is shown in FIG. 1. There, a plurality of agents 110–160 communicates over an external bus 170 according to a predetermined bus protocol. “Agents” may include general-purpose processors 110–140, memory controllers 150, interface chipsets 160, input output devices and/or other integrated circuits (not shown) that process data requests. The bus 170 may permit several external bus transactions to be in progress at once.
In multi-agent systems, the bandwidth of the external bus 170 can define a limit to system performance. Clock speeds within an agent typically are much faster than clock speeds of the external bus. A processor core (not shown) for example can issue many data requests (read requests and write requests) in the time that the external bus 170 can execute a single request. Further, an agent must share the external bus 170 with other agents. These factors can introduce unwanted latency to the processing of data requests within an agent.
Not all data requests are created equal. Currently, Intel Corporation, the assignee of the present invention, is designing an agent that will process core read requests, prefetch requests and write requests. Core read requests are requests for addressed data to be read to the agent's processing core (“core”). Typically, core read requests identify data for which the agent has an immediate need. Prefetch requests, by contrast, refer to data that is likely to be used by the core in the not-so-distant future. By prefetching the data into the agent prior to the time the core actually issues a read request for it, the data should be available to the core in an internal cache. The internal cache may operate at a faster clock rate than the external bus and, therefore, may satisfy the expected core request with reduced latency. Write requests typically identify data that is being returned by the agent to system storage. The data may be evicted because the agent is no longer using it and new data is being read to a memory location that the evicted data occupied. Other data requests may be associated with other priorities.
Given the bandwidth limitations of an external bus and the relative priorities observed with respect to the different data requests handled within an agent, the inventors determined that there is a need in the art for a bus control algorithm that schedules requests to be posted on the external bus according to a predetermined priority scheme.